Process for purifying an organic acid

ABSTRACT

A process is disclosed for purifying an aqueous feed stream comprising a product organic acid, such as lactic acid, and a strong contaminant, such as pyruvic acid or oxalic acid. The molar concentration of the product organic acid in the feed stream typically is at least 20 times greater than the molar concentration of the strong contaminant. The aqueous feed stream is contacted with a first immiscible basic extractant that has at least a 3-fold greater affinity for the strong contaminant than for the product organic acid. The majority of the strong contaminant and some product organic acid become complexed with the first immiscible basic extractant. The complexed first immiscible basic extractant is separated from the aqueous stream, thereby producing a first effluent stream that comprises product organic acid and that has a greater ratio of molar product organic acid to molar strong contaminant than the aqueous feed stream did. The complexed first immiscible basic extractant is contacted with a displacing acid. The first immiscible basic extractant has a greater affinity for the displacing acid than it does for the strong contaminant or the product organic acid, and as a result, product organic acid and strong contaminant are displaced over a period of time from the complexed first immiscible basic extractant, producing a second effluent stream that comprises a major amount of product organic acid and a third effluent stream that comprises a major amount of strong contaminant.

BACKGROUND OF THE INVENTION

The present invention relates generally to processes for producingorganic acids, such as lactic acid.

Lactic acid has a number of commercial uses, for example in foodmanufacturing, pharmaceuticals, plastics, textiles, and as a startingmaterial in various chemical processes. In addition, it is used in themanufacture of polylactic acid, a degradable plastic.

Although organic acids can be prepared by chemical synthesis, productionby fermentation is generally less expensive. It is well known to producelactic acid by fermentation using microorganisms such as Lactobacillusdelbrueckii. The broth that results from fermentation containsunfermented sugars, carbohydrates, amino acids, proteins, and salts, aswell as organic acids, such as lactic acid. Typically, the organic acidis recovered from the fermentation broth and undergoes furtherpurification before it is used. Purified organic acids recovered fromfermentation broths can comprise small amounts of impurities, such asstrong acids or certain unknown compounds. Some of these impurities cancause an undesirable color or can interfere with downstream processingof the organic acid. For example, lactic acid as it is sold commerciallytypically comprises small amounts of impurities such as pyruvic acid andoxalic acid. Even though present in relatively small amounts, suchimpurities can have negative effects on polymers produced from thelactic acid. For example, when lactic acid is polymerized to producepolylactic acid (PLA), the presence of even small amounts of pyruvicacid can cause the polymer to have an undesirable yellow color. However,it is difficult to further purify lactic acid that contains only a smallfraction of pyruvic acid in the first instance.

Thus, there is a need for improved processes for the production andrecovery of relatively pure organic acids, particularly lactic acid.

SUMMARY OF THE INVENTION

One aspect of the present invention is a process for purifying anaqueous feed stream that comprises a desired product organic acid and atleast one strong contaminant. In certain embodiments, the aqueous feedstream can comprise a fermentation broth or can be obtained from afermentation broth. (Whenever an acid is referenced herein, either asthe desired product or as a contaminant, it should be understood thatsome or all of the acid may be present in the form of salts.) The molarconcentration of the product organic acid in the feed stream can be atleast 10 times greater than the molar concentration of the strongcontaminant, and more preferably the molar concentration of the productorganic acid to the strong contaminant is at least 20. In certainembodiments the molar concentration of the product organic acid to thestrong contaminant is at least 90, in certain embodiments it is at least500 and in certain embodiments it is at least 1000. The aqueous feedstream is contacted with a first immiscible basic extractant that has aselectivity, under the existing process conditions (including thecombination of acids, solvents, etc., that are present) for the strongcontaminant relative to the product organic acid that is greater than 3.The selectivity, which is further defined below, is preferably greaterthan 15, more preferably greater than about 25, most preferably greaterthan about 100. Preferably the selectivity is greater than the ratio ofproduct organic acid to strong contaminant in the feed.

The contacting step in which the aqueous feed stream is contacted with afirst immiscible basic extractant is preferably performed withsufficient equilibrium or near equilibrium stages, and with sufficientquantity of the first immiscible basic extractant (such as a solid amineion exchanger or liquid amine extractant) to remove the majority of thestrong contaminant. In certain embodiments the first immiscible basicextractant has previously been used to treat a solution comprising theproduct organic acid and at least one weak contaminant (e.g., it isrecycled).

As a result, the majority of the strong contaminant and less than about33 wt % of the product organic acid become complexed with the firstimmiscible basic extractant. “Majority” as used herein means more than50% by weight of the substance, in this case the strong contaminant,that is present. In other words, more than 50% by weight of the strongcontaminant present in the feed complexes with the extractant. Thecomplexed first immiscible basic extractant is separated from theaqueous stream, thereby producing a first effluent stream that comprisesproduct organic acid and that has a greater ratio of product organicacid to strong contaminant than the aqueous feed stream did. Thecomplexed first immiscible basic extractant is contacted with adisplacing acid. The first immiscible basic extractant has a greateraffinity for the displacing acid than it does for the strong contaminantor the product organic acid, and as a result, product organic acid andstrong contaminant are displaced over a period of time from thecomplexed first immiscible basic extractant. This produces a secondeffluent stream that comprises a major amount of product organic acid(i.e., more than 50% by weight of the solids dissolved or suspended inthe stream are the product organic acid) and a third effluent streamthat comprises a major amount of strong contaminant. Preferably, thetotal amount of product organic acid present in the first effluentstream and in the second effluent stream is at least about 90% by weightof the product organic acid that was present in the feed stream. Morepreferably, at least about 98% by weight of the product organic acid isrecovered in those streams.

In many embodiments of the process, the strong contaminant comprises anorganic acid that has a pK_(a) that is lower than the pK_(a) of theproduct organic acid. If the desired product organic acid is lacticacid, the strong contaminant preferably has a pK_(a) less than about3.46. In certain specific embodiments of the process involving a basicextractant that is a solid ion exchange resin, the strong contaminant isselected from the group consisting of pyruvic acid, oxalic acid,citraconic acid, citric acid, and mixtures thereof.

In other embodiments, the strong contaminant can be a weaker acid (e.g.,higher pK_(a)) than the desired organic acid product, but can havegreater hydrophobic and/or hydrogen bonding character than the product.The strong contaminant is selectively removed relative to the organicacid of interest by an immiscible basic extractant comprising a solventor a solvent mixture, for example an amine mixture comprising 1 Mtrilaurylamine and 1 M dodecanol with dodecane as a diluent.

When the immiscible basic extractant comprises a solvent mixture,preferably the organic acid of interest has somewhat hydrophobic orstrong hydrogen bonding characteristics, and the strong contaminant musteither (1) be of similar H-bonding and/or hydrophobic character andlower pK_(a) than the acid of interest (acidic low pK_(a) species) or(2) have a sufficiently stronger H-bonding and/or hydrophobic characterso that the strong contaminant can still be removed despite its having ahigher relative pK_(a). Thus, if the strong contaminant has a lowerpK_(a) than the organic acid product, a solid ion exchange resin can beused as an extractant to remove the strong contaminant. For example, ifthe organic acid to be recovered is lactic acid, strong contaminantsthat can be removed by methods of the present invention involving ionexchange resins include HCl, H₂SO₄, pyruvic acid and oxalic acid, amongothers. Acetic acid and butyric acid do not, however, have significantlylower pK_(a) values than lactic acid, and they are not as readilyremoved by ion exchange resins. However strong contaminants havingeither low pK_(a) or high hydrophobicity/hydrogen bondingcharacteristics relative to the organic acid product, can be removedusing extractants that are solvents or a solvent mixture. For example,pyruvic acid, H₂SO₄, and butyric acid can be removed from an aqueousfeed stream comprising lactic acid with use of an amine solventextractant of the present invention. Although the first immiscible basicextractant can take various forms, one that is preferred is a weak baseion exchange resin. Preferably the weak base ion exchange resincomprises a tertiary amine moiety. One of the advantages of manyembodiments of this process is the ability to further purify a streamthat already contains a very low percentage of impurities. For example,in one embodiment, the molar concentration of lactic acid, e.g., theproduct organic acid, in the feed stream is at least 20 times greaterthan the molar concentration of the strong contaminant in the feedstream, and the selectivity is greater than about 25. In anotherembodiment, the molar concentration of the lactic acid, e.g., theproduct organic acid, in the feed stream is at least 300 times greaterthan the molar concentration of the strong contaminant in the feedstream, and the selectivity is greater than about 500.

In one embodiment of the process, the feed stream, the first effluentstream, and the second effluent stream further comprise a weakcontaminant. For example, where the product organic acid is lactic acid,the weak contaminant can be an organic acid having a pK_(a) greater thanabout 4.26, such as propionic acid, butyric acid, malonic acid, succinicacid, or mixtures thereof. In this situation, the process can furthercomprise the steps of combining the first effluent stream and the secondeffluent stream to form a combined product organic acid stream, and thencontacting the combined product organic acid stream with a secondimmiscible basic extractant. The majority of the product organic acidbecomes complexed with the second immiscible basic extractant.Preferably, at least 90% by weight of the product acid becomescomplexed, more preferably at least 95%. The complexed second immisciblebasic extractant can then be separated from the stream, therebyproducing a fourth effluent stream that comprises the majority of theweak contaminant that was present in the combined product organic acidstream. Preferably the second immiscible basic extractant comprises aweak or strong base ion exchange resin.

Optionally, this embodiment of the process can further comprisecontacting the fourth effluent stream with a third immiscible basicextractant that has a greater affinity for the product organic acid thanfor the weak contaminant, whereby the majority of the product organicacid that is present in the fourth effluent stream becomes complexedwith the third immiscible basic extractant. The complexed thirdimmiscible basic extractant can then be separated from the stream,thereby producing a fifth effluent that comprises the majority of theweak contaminant that was present in the combined product organic acidstream. Then the complexed second immiscible basic extractant and thecomplexed third immiscible basic extractant can be contacted with one ormore displacing acids, thereby displacing product organic acid therefromin one or more additional effluent streams.

In another variation of the process, the third effluent stream iscontacted with an additional immiscible basic extractant that has agreater affinity for the strong contaminant than for the product organicacid. As a result, the majority of the strong contaminant present in thethird effluent stream becomes complexed with the additional immisciblebasic extractant. The complexed additional immiscible basic extractantis separated from the remaining stream, thereby producing an additionaleffluent that comprises the majority of the product organic acid thatwas present in the third effluent.

One particularly preferred embodiment of the invention is a process forpurifying lactic acid. The embodiment involves providing an aqueous feedstream comprising lactic acid (defined herein to include any saltsthereof) and at least one strong contaminant acid having a pK_(a) lessthan about 3.46. The molar concentration of lactic acid in the feedstream is at least 20 times greater than the molar concentration of thestrong contaminant acid. The aqueous feed stream is contacted with afirst basic ion exchanger that has a greater affinity for the strongcontaminant acid than for lactic acid, such that the majority of thestrong contaminant acid and some lactic acid become complexed with thefirst basic ion exchanger. The complexed first basic ion exchanger isseparated from the aqueous stream, producing a first effluent streamthat comprises lactic acid and that has a greater ratio of lactic acidto strong contaminant acid than the aqueous feed stream did. Thecomplexed first basic ion exchanger is contacted with a displacing acid,such as HCl, H₂SO₄, or H₃PO₄, and the first basic ion exchanger has agreater affinity for the displacing acid than it does for the strongcontaminant acid or lactic acid. Lactic acid and strong contaminant acidare displaced over a period of time from the complexed first basic ionexchanger, producing a second effluent stream that comprises a majoramount of lactic acid, and a third effluent stream that comprises amajor amount of strong contaminant acid.

In some embodiments of this process, the strong contaminant acid isselected from the group consisting of pyruvic acid, oxalic acid,citraconic acid, citric acid, and mixtures thereof. In certainembodiments of the process, the molar concentration of lactic acid inthe feed stream is at least 100 times greater than the molarconcentration of the strong contaminant acid in the feed stream and theselectivity is greater than about 250. In certain embodiments, the ratiois at least 300 and the selectivity is greater than about 500.

Preferably, the first basic ion exchanger has an affinity for thedisplacing acid that is at least 10 times greater than its affinity forpyruvic acid.

Preferably, the first effluent stream and the second effluent streamcollectively have a ratio of lactic acid to strong contaminant that isgreater than 300. More preferably, they collectively have a ratio ofmolar lactic acid to molar strong contaminant that is greater than about1,000. In some embodiments of the process, at least 90%, and morepreferably 98% by weight of the lactic acid present in the feed streamis recovered in the first effluent stream and the second effluentstream.

In a particular embodiment of the process, the aqueous feed streamcomprises no more than about 0.15 moles of cations selected from thegroup consisting of Ca, Mg, Na, Fe, Zn, Zr, and Li, per mole of lacticacid; no more than about 0.05 moles of anions selected from the groupconsisting of Cl, SO₄, PO₄, and NO₃, per mole of lactic acid; nor morethan about 0.03 mole of strong acid contaminants selected from the groupconsisting of pyruvic acid, oxalic acid, citraconic acid, and citricacid, per mole of lactic acid; and no more than about 0.02 mole of weakacid contaminants selected from the group consisting of propionic acid,butyric acid, malonic acid, and succinic acid, per mole of lactic acid.

Another aspect of the invention is a process for purifying lactic acidinvolving providing an aqueous fermentation broth comprising lactic acid(defined herein to include any salts thereof) and pyruvic acid (alsodefined herein to include any salts thereof). The molar concentration ofthe lactic acid is at least 20 times greater than the molarconcentration of pyruvic acid in the aqueous fermentation broth. Cellsare removed from the broth to form an aqueous feed stream. Any methodknown in the art for removing cells can be used (e.g., centrifugation orfiltration, among others). The aqueous feed stream is contacted withmeans for complexing pyruvic acid, and the means has a greater affinityfor pyruvic acid than for lactic acid, so that the majority of thepyruvic acid and some lactic acid form complexes therewith. Thecomplexes are separated from the aqueous stream, thereby producing afirst effluent stream that comprises lactic acid and that has a greaterratio of lactic acid to pyruvic acid than the aqueous feed stream did.The complexes are contacted with means for displacing lactic acid andpyruvic acid therefrom, thereby producing a second effluent stream thatcomprises a major amount of lactic acid and a third effluent stream thatcomprises a major amount of pyruvic acid.

In certain embodiments the first immiscible basic extractant is a solidbasic extractant, and the aqueous feed stream is contacted with thefirst immiscible basic extractant in a packed bed. Preferably inembodiments involving a packed bed, the ratio of molar concentration ofproduct organic acid to molar concentration of strong contaminant in thesecond effluent stream is within about 10% of the selectivity.

In other embodiments, the first immiscible basic extractant is a liquidbasic extractant, and the aqueous feed stream is contacted with thefirst immiscible basic extractant in a multistage process. Preferablythe ratio of product organic acid to strong contaminant is within about10% of the selectivity.

Furthermore, in certain embodiments the aqueous feed stream comprisesmore than one strong contaminant, and at least one of the strongcontaminants is a displacing acid.

Certain embodiments are directed to a process involving an aqueous feedstream that comprises a desired product organic acid, at least onestrong contaminant, and a weak contaminant. The first effluent streamalso comprises the weak contaminant, and the first effluent stream isprocessed by at least one of extraction or distillation (by methodsknown in the art), whereby at least two fractions are produced, apurified product organic acid fraction comprising between about 90% and99.5% by weight of the product organic acid that was present in the feedstream, and a weak contaminant fraction comprising product organic acidand weak contaminant. The ratio of molar concentration of productorganic acid to molar concentration of weak contaminant in the weakcontaminant fraction is less than the ratio of molar concentration ofproduct organic acid to molar concentration of weak contaminant in thefeed stream. The weak contaminant fraction is contacted with a thirdimmiscible basic extractant that has a selectivity for the productorganic acid relative to the weak contaminant that is greater than about3, and the majority of the product organic acid and less than about 33wt % of the weak contaminant become complexed with the third immisciblebasic extractant. The complexed third immiscible basic extractant isseparated from the aqueous stream, to produce an effluent stream thatcomprises weak contaminant. The effluent stream that comprises weakcontaminant has a greater ratio of weak contaminant to product organicacid than the aqueous feed stream did. The complexed third immisciblebasic extractant is contacted with a displacing acid, and the thirdimmiscible basic extractant has a greater affinity for the displacingacid than it does for the product organic acid or the weak contaminant.The displacing acid is present in sufficient amount to cause productorganic acid and weak contaminant to be displaced over a period of timefrom the complexed third immiscible basic extractant to produce a weakcontaminant effluent stream that comprises a major amount of weakcontaminant and a product organic acid stream that comprises a majoramount of the product organic acid. Preferably the purified productorganic acid fraction comprises between about 95% by weight and 99.5% byweight of the product organic acid that was present in the feed stream.

Certain embodiments are directed to a process for purifying an organicacid that comprises providing an aqueous feed stream comprising aproduct organic acid and one strong contaminant. The molar concentrationof the product organic acid is at least 20 times greater than the molarconcentration of the strong contaminant. The aqueous feed stream iscontacted with a first immiscible basic extractant that has aselectivity for the strong contaminant relative to the product organicacid that is greater than about 3. The majority of the strongcontaminant and less than about 33 wt % of the product organic acidbecome complexed with the first immiscible basic extractant. Thecomplexed first immiscible basic extractant is separated from theaqueous stream, thereby producing a first effluent stream that comprisesproduct organic acid and that has a greater ratio of product organicacid to strong contaminant than the aqueous feed stream did. Thecomplexed first immiscible basic extractant is exposed to at least oneof (1) a change in temperature, (2) a change in solvent concentration,or (3) a change in displacing agent concentration. The exposure causesproduct organic acid and strong contaminant to be displaced over aperiod of time from the complexed first immiscible basic extractant,thereby producing a second effluent stream that comprises a major amountof product organic acid and a third effluent stream that comprises amajor amount of strong contaminant. The change in solvent concentrationcan be achieved using methods known in the art, such as evaporationsolvents present in the aqueous feed stream. Furthermore combinations oftemperature change, solvent change, and/or displacing acid or baseconcentration can be used to obtain selective release of desired productor of the impurities from the complexed first immiscible basicextraction. The displacing agent can be either a displacing acid asdiscussed above, or a displacing base. A base, such as NaOH, can be usedas a displacing agent, and the displaced material can then be treatedusing methods known in the art to recovered the desired product organicacid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram of one embodiment of the presentinvention.

FIG. 2 is a process flow diagram of another embodiment of the presentinvention, comprising steps that can be performed in addition to thoseshown in FIG. 1.

FIG. 3 is a process flow diagram of yet another embodiment of thepresent invention, comprising steps that can be performed in addition tothose shown in FIG. 1.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The process of the present invention can be used to recover and purify avariety of organic acids. The process is especially well suited forrecovery and purification of a monocarboxylic, dicarboxylic, ortricarboxylic acid having from 2 to 8 carbon atoms. Preferably, theorganic acid is a hydroxy organic acid (or a mixture of two or more suchacids). The hydroxy organic acid can be an alpha, beta, delta, gamma, orepsilon hydroxy acid. Most preferably, the organic acid is lactic acid.

The process of the present invention can recover purified organic acidfrom a fermentation broth. However, the process is also suitable for usein purifying organic acids from other sources, such as lactic acid ofcommerce. “88% lactic acid” and “lactic acid of commerce” refer to atypical commercially available lactic acid, which is actually a mixtureof monomeric lactic acid, linear dimer lactic acid or lactoyl lacticacid, short chain lactic acid oligomers, water, a small quantity ofcyclic dimer lactic acid or lactide, and small amounts of impurities.When commercial lactic acid is diluted in a large excess of water, thedimers and oligomers slowly hydrolyze or convert to the monomeric formof lactic acid. When concentrated lactic acid is diluted with water to a50 wt % concentration, dimers and oligomers initially present willslowly hydrolyze to a mixture that is largely monomeric lactic acid, butwhich can still contain about 3 to 4 wt % dimer lactic acid, and traceamounts of higher oligomers.

FIG. 1 shows an embodiment of the process for recovering lactic acidfrom a fermentation broth. A seed train 10 is fed to a fermentationvessel 12 that contains fermentation medium. The fermentation producesan aqueous broth that comprises the desired organic acid, in this caselactic acid. (Whenever an organic acid is referred to in this patent,including in the claims, it should be understood that some or all of theacid can be present in the form of salts. For the sake of brevity, anyreference to the acid herein covers either form (free acid or salt), ora mixture of both.) The broth also contains one or more strongcontaminants, in this case pyruvic acid, as well as unfermented sugarsand other impurities. In some embodiments of the process, the ratio ofthe desired product acid (e.g., lactic acid) in molar concentration tomolar concentration of the strong impurity (e.g., pyruvic acid) isgreater than about 300. In certain embodiments the ratio of the desiredproduct acid (e.g., lactic acid) in molar concentration to molarconcentration of the strong impurity (e.g., pyruvic acid) is greaterthan about 500, and in certain embodiments its greater than 1000.

A “strong contaminant” as used herein means a chemical species, such asan organic acid, for which the immiscible basic extractant selected foruse has at least a 3 fold greater selectivity or affinity than for theproduct of interest.

For embodiments of the present invention in which a solid immisciblebasic extractant comprising an anion exchange resin is used, a strongcontaminant that is to be removed from an organic acid aqueous feedstream can have a lower pK_(a) value than that of the organic acid thatis to be recovered from the feed stream.

For embodiments of the present invention involving liquid immisciblebasic extractants comprising a solvent, a strong contaminant either (a)has a lower pK_(a) value than that of the organic acid that is to berecovered from the feed stream, or (b) the strong contaminant has asimilar or higher pK_(a) value than the organic acid of interest, but ismore hydrophobic or has more hydrogen bonding groups or both.

For example, for the case of solid ion exchange extractants, oxalic acid(pK_(a)=1.27) and pyruvic acid (pK_(a)=2.48) are strong contaminants ina solution of lactic acid (pK_(a)=3.86). Examples of other strongcontaminants that can be present in a lactic acid feed stream includecitraconic acid, citric acid, and other organic acids with pK_(a) valuesless than about 3.46. Weak contaminants that are not removed by thesolid ion exchange extractant can include non-acidic species such asglucose, and weak acids such as acetic acid, succinic acid, and butyricacid.

As an example for the case of liquid amine based immiscible extractantsat 20-30 deg C. used to recover lactic acid from a solution, strongcontaminants can include both strong acids, such as above, as well ashydrophobic weak acids. Examples of hydrophobic weak acids that can beremoved by liquid immiscible extractants are n-butyric acid, iso-butyricacid, and cyclic weak aromatic acids, such as benzoic acid, amongothers.

In some embodiments of the process, the feed may comprise one or morestrong contaminants of unknown chemical composition. For example,certain lactic acids sold commercially comprise contaminants of unknownchemical composition that have apparent pK_(a) values less than about3.46. Using the present invention, a feed stream comprising such alactic acid can be purified to have a lower concentration of some ofthese unknown contaminants. The broth 14 is withdrawn from thefermentation vessel 12. Cells in the broth can be separated, for exampleby filtration or centrifugation, and removed as a waste stream 16.Optionally, the broth can be further purified by removing strong anionsand/or cations. Strong anions such as chloride, sulfate, phosphate, andnitrate can be selectively removed from neutral or acidic pH streamscontaining organic acid at high selectivity. Strong cations that can beremoved include Ca, Mg, K, Na, Fe, Zn, Zr, and Li cations, among others.For example, the fermentation broth can be contacted with a cationexchanger (e.g., strongly acidic cation exchanger), an anion exchanger(e.g., weakly basic anion exchanger), or both sequentially.

The broth 14 is then contacted with a first immiscible basic extractantin step 18. Preferably, this is done using counter-current flow. Amixer-settler apparatus can be used, among other possibilities. Thisextractant is “immiscible” in that it does not mix with the broth, butthe extractant may or may not be liquid. For example, the extractant cancomprise an amine compound that has the ability to form complexes withone or more of the organic acids present. In particular, this firstextractant should have an affinity for the strong contaminant (pyruvicacid) that is greater than its affinity for the desired product (lacticacid). “Affinity” as used herein means the tendency to complex withanother species, such as lactic acid or pyruvic acid, under the existingprocess conditions, including the particular combination of acids,solvents, and other ingredients that are present. Equal affinity wouldmean that when contacted with a solution of 50% lactic acid and 50%pyruvic acid, the extractant would complex with equal amounts of the twoacids. Because the feed solutions in the present invention will oftencomprise a high ratio of lactic acid to pyruvic acid, the extractant'saffinity for the contaminant (pyruvic acid) should be much greater thanits affinity for lactic acid. Preferably, the extractant's affinity forpyruvic acid is at least about 20 times greater than its affinity forlactic acid.

The amine liquid extractant can comprise a primary, secondary ortertiary amine. In certain embodiments, the amine is an alkylamine inwhich the aggregate number of carbon atoms is from about 4 to 36.Specific examples include n-butylamine, tri-n-butylamine, octylamine,tri-n-octylamine, di-decylamine, dodecylamine, and tri-dodecylamine(also called trilaurylamine), among others. The amine should beimmiscible with the aqueous feed solution such that two phases areformed. A liquid extractant of the present invention can optionally alsocomprise a diluent and/or an enhancer. A diluent can be used as acomponent of the basic immiscible extractant to lower its viscosity, orto increase the selectivity of the extractant against other unwantedspecies, among other reasons. Suitable diluents include, for example,pure or mixed aromatic or aliphatic hydrocarbons, such as xylene,toluene, decane, dodecane, kerosene, and mixtures thereof. “Enhancer”refers to a chemical species that acts to enhance the performance of thebasic immiscible extractant. The enhancer can strengthen the basicimmiscible extractant:strong contaminant complex or immiscibleextractant:organic acid complex and/or help solubilize the complex.Examples of suitable enhancers include polar species selected fromalcohols, diols, ketones, diketones, fatty acids, chlorinated species,and other species known in the art.

Alternatively, the first basic extractant can comprise a basic ionexchange resin. The ion exchange resin should have a greater affinityfor the strong contaminant than it does for the desired product, asdescribed above. Suitable ion exchange resins include pyridine resins,imidazole resins, and tertiary amine resins, among others. One specificexample is Ionac A365 (Sybron Chemicals, Birmingham, N.J.). Strong baseand weak base ion exchangers can also be used. It is preferred that theamount of the first extractant used in the process be sufficient tocreate an overall complexing capacity that is greater than theoreticallyneeded to complex with all of the pyruvic acid or other strongcontaminant present. In this way, despite the fact that some lactic acidwill be complexed as well, the fraction of pyruvic acid complexed can bemaximized.

The first basic extractant, which has a greater affinity for pyruvicacid than for lactic acid, forms complexes with the majority of thepyruvic acid that is present in the broth. Due to the relatively largeconcentration of lactic acid in the broth, the extractant also formssome complexes with lactic acid, despite its lower affinity for thatacid. Thus, with the majority of the pyruvic acid removed, a firsteffluent stream 20 is produced that has a higher ratio of lactic acid topyruvic acid (M lactic acid:M pyruvic acid) than the original broth 14.

The first basic extractant can then be separated from most of thepyruvic and lactic acid in the complex by an acid displacement step 24.A stream 26 comprising an aqueous solution of a displacing acid, such asHCl, H₃PO₄, oxalic acid, H₂SO₄ or trifluoroacetic acid, is contactedwith the first extractant, which at this point is still complexed withpyruvic and lactic acid. Preferably, the displacing acid has a pK_(a) ofabout −2 to 1.8. The displacing acid can also be present in a mixturewith other organic acids and species, such as a mixture of HCl, H₂SO₄,lactic acid and acetic acid. Preferably the concentration of thedisplacing acid is from between about 1 and 40%, more preferably from 2to 5% for H₂SO₄ and from 20 to 30% for mixtures containing variousacids. In the case of mixtures of various acids, only the acids having apK_(a) of about −2 to 1.8 present in that mixture will act as displacingacids.

Because the extractant (i.e., ion exchanger) has a greater affinity forthe displacing acid than for either lactic acid or pyruvic acid, thelatter two acids are displaced from the complexation sites. Since theextractant has a lower affinity for lactic acid than for the other twoacids, lactic acid tends to be displaced first, and is removed as alactic acid rich-second effluent stream 28. This lactic acid-rich streamoptionally can be combined with the first effluent stream 20 to form acombined lactic acid product stream. Preferably, this stream comprisesat least about 98% by weight of the lactic acid that was present in thefeed, and less than about 20 ppm each of pyruvic acid and oxalic acid,more preferably less than about 10 ppm, most preferably less than about2 ppm.

After the majority of the lactic acid has been displaced, the pyruvicacid starts to be displaced in greater quantities. This thereforegenerates a pyruvic acid-rich third effluent stream 30, which optionallycan be purified for use in processes that require that particular acid.

A stream 32 is generated that comprises the first extractant, which isnow complexed with the displacing acid. This stream 32 is subjected to abasic regeneration step 34, in which a stream 36 comprising a base suchas 5% aqueous solution of NaOH is contacted with the complexed firstextractant. The base displaces the displacing acid from the extractant,thus creating a stream 38 of regenerated first extractant, which can berecycled 42 for use in the step 18. This operation also produces astream 40 comprising regenerating acid, which optionally can be recycledfor use in stream 26.

Other methods of regenerating the first extractant could be used aswell.

FIG. 2 shows optional downstream process steps that are well suited foruse when the original feed to the process comprises a weak contaminantas well as a strong contaminant.

A “weak contaminant” as that term is used herein relates to embodimentsof the present invention involving solid amine extractants, and means achemical compound that has a pK_(a) value greater than that of theorganic acid that is to be recovered.

A “weak contaminant” as that term is used herein for the situation ofliquid amine mixtures means a chemical compound that has a pK_(a) valuegreater than that of the organic acid that is to be recovered andtypically is not highly hydrophobic nor prone to forming strong hydrogenbonds with various components of the liquid amine mixture.

For example, for solid based amines, acetic acid, which has a pK_(a)value of 4.76, can be a weak contaminant present in a feed streamcomprising lactic acid. Other examples of weak contaminants thatcommonly are present in a lactic acid feed stream, especially oneobtained from a fermentation broth, include propionic acid, butyricacid, acetic acid, malonic acid, succinic acid, and other organic acidshaving a pK_(a) greater than about 4.26.

For example, for liquid extractants that are mixtures comprisingimmiscible amines, acetic acid, which has a pK_(a) value of 4.76, can bea weak contaminant present in a feed stream comprising lactic acid.However, in this case butyric acid is not a weak contaminant, relativeto lactic acid, in typical solvents, due to its hydrophobic character.

In FIG. 2, the first effluent stream 20 and the second effluent stream28 are combined to form a combined lactic acid product stream 50. Thecombined product stream is then contacted in step 52 with a secondimmiscible basic extractant. The second immiscible basic extractant canbe, for example, a weak base ion exchange resin, such as Amberlite IR35,comprising a tertiary amine moiety. This second extractant preferablyhas a greater affinity for lactic acid than for acetic acid . Inaddition, the amount of this extractant present should be more thansufficient to complex with essentially all of the lactic acid present inthe steam. Therefore, the second extractant forms complexes primarilywith lactic acid, and to a much smaller degree with acetic acid. Thecomplexes are separated from the remaining liquid as part of stream 56,thus leaving a fourth effluent stream 54.

The fourth effluent 54 is contacted with a third immiscible basicextractant, which is preferably a basic ion exchange resin. Suitableresins include Amberlite IR 35. This third extractant preferably has agreater affinity for lactic acid than for pyruvic acid. Therefore, mostof the lactic acid in the fourth effluent complexes with the ionexchanger, and the complexes are removed in stream 70. A fifth effluentstream 68 is generated which is rich in the weak contaminant.

The streams 56 and 70 comprise complexes of the second and thirdextractants with primarily lactic acid. The lactic acid can therefore berecovered by contacting these complexes with streams 58 and 72 ofdisplacing acid. Since the second and third extractants have greateraffinity for the displacing acid than for lactic acid, the latter isdisplaced into additional effluent streams 62 and 76, from which it canbe recovered. These streams 62 and 76 preferably comprise more thanabout 90% by weight of the lactic acid that was present in the combinedstream 50, more preferably at least about 95%.

The streams 64 and 78 comprise the ion exchangers complexed withdisplacing acid, which can subsequently be regenerated by contact with abase (not shown in FIG. 2), and then can be recycled for further use inthe process.

FIG. 3 shows another set of optional steps that can be performed inaddition to those shown in FIG. 1. This variation of the process isespecially useful when the ratio of lactic acid to pyruvic acid (Molarlactic acid:Molar pyruvic acid) in the original feed is greater thanabout 300. In this variation of the process, the third effluent stream30, which is rich in pyruvic acid, is contacted with an additionalimmiscible basic extractant in the step 80. This extractant has agreater affinity for the pyruvic acid than for lactic acid, andpreferably is an ion exchange resin such as Amberlite IR-35. Therefore,the extractant complexes primarily with pyruvic acid, and thesecomplexes are removed in stream 84. An additional lactic-acid richeffluent stream 82 is generated, from which lactic acid can berecovered.

The stream 84 comprising complexes of ion exchanger and primarilypyruvic acid is then contacted in step 86 with a stream 85 comprising adisplacing acid, such as HCl, 4% solution in water. The extractant has agreater affinity for the displacing acid than for the pyruvic acid, andtherefore the latter is displaced as the former complexes with theextracted. The pyruvic acid is removed in effluent stream 88, while astream 90 comprising the complexes of the displacing acid proceeds to aregeneration step 92, in which the complexes are treated with a basestream 94. The result is a regenerated resin stream 98 that can berecycled for further use in the process, and an effluent stream 96 thatcomprises the displacing acid, which can also be recycled.

The process embodiments described above achieve high selectivity, andthus are highly effective in removing contaminants from an organic acidsolution or suspension, even if it is relatively pure in the firstinstance. This ability to remove selectively an impurity that is presentat a low concentration is a major advantage of many embodiments of thepresent invention. “Selectivity” as used herein refers to the apparentor effective selectivity of the extractant under the process conditions,including the concentrations of desired product acid, contaminants,solvents, and other ingredients to which the extractant is exposed,which ingredients may be present in more than one liquid phase. Theselectivity (S) often is slightly less than the theoretical selectivitydue to contacting constraints, among other reasons. Theoreticalselectivity can be expressed in terms of the following equation:${{theoretical}\quad {selectivity}} = \frac{\frac{\begin{matrix}\text{amount~~of~~strong~~contaminant~~complexed} \\\text{with~~immiscible~~basic~~extractant}\end{matrix}}{\begin{matrix}\text{amount~~of~~organic~~acid~~complexed} \\\text{with~~immiscible~~basic~~extractant}\end{matrix}}}{\frac{\begin{matrix}\text{amount~~of~~strong~~contaminant~~in} \\\text{effluent~~stream}\end{matrix}}{\text{amount~~of~~organic~~acid~~in~~effluent~~stream}}}$

The amount referred to in the equation can be any quantitativemeasurement such as the area of a gas chromatography peak or a HPLCpeak, moles, and grams, among others. The selectivity will be greaterthan about 1, and preferably much greater than 1. Preferably, where thedesired product is lactic acid, the selectivity is at least about 10.Preferably, the selectivity is at least 80% of the theoreticalselectivity. More preferably the selectivity is at least about 90% to95% of the theoretical selectivity, and most preferably at least about99% of the theoretical selectivity.

Preferably, the ratio of the molar concentration of the organic acid tothe molar concentration of the strong contaminant in the feed stream isat least about equal to the selectivity for the strong contaminant. Ifthe feed stream comprises lactic acid, preferably the ratio of M lacticacid to M pyruvic acid (the strong contaminant) is greater than about18. In certain embodiments of the process, the ratio of organic acid tostrong contaminant (M organic acid:Molar strong contaminant) is greaterthan the selectivity (S) for the strong contaminant and less than thesquare of the same selectivity (S²). In still other embodiments of theprocess, the ratio in the feed is even greater than S².

FIGS. 1-3 illustrate certain specific combinations of steps that can beused, but those skilled in the art will recognize that there would bemany ways to implement the present invention. For example, the processcan be operated in a batch, continuous, or semi-continuous manner.Certain embodiments of the invention can be further understood from thefollowing examples.

EXAMPLE 1 Selectivity of Resin IRA-35 for Pyruvic Acid Relative toExcess Lactic Acid

TABLE 1 Feed Equilibrium Conc mole/liter Solution Liquor Resin K_(d)Selectivity Pyruvic Acid 0.005 0.001 0.024 26.9115 14.69 Lactic Acid0.503 0.386 0.706 1.832 Lactic Dimer 0.006 0.003 0.019 6.715 3.67

A feed solution of 3 ml was prepared as shown in Table 1 containinglactic acid and pyruvic acid in water. The feed was 45.3 gram/literlactic acid and 440 mg/liter pyruvic acid, representing a typicalfermentation broth stream. This was contacted with a weak base anionresin, IRA-35, at 20 deg C., in a single stage batch equilibrationexperiment. A selectivity of 14.69 for pyruvic acid relative to lacticacid was observed. A total of 81.8% of the pyruvic acid was removed fromthe solution onto the ion exchange resin in this single stage batchexperiment, whereas only 23.4% of the lactic acid was removed.

EXAMPLE 2 Purification of a Lactic Acid Feed Stream Comprising TraceAmounts of Pyruvic Acid and Oxalic Acid Using a Packed Column(Multistage) Contactor With Continuous Flow

Three feed solutions were prepared to simulate fermentation brothscomprising lactic acid. The first feed solution (FS1) was made combining272.7 g 88% L-lactic acid (Pfansteihl), 2.572 g 95% pyruvic acid(Aldrich), 20.868 g 10% w/v oxalic acid (LabChem, Inc.), and deionizedwater to a volume of 4 liters. Similarly, a second feed solution (FS2)was prepared comprising 270.4 g 88% L-lactic acid, 2.424 g 95% pyruvicacid, 20.756 g 10% w/v oxalic acid, and deionized water in sufficientamount to attain a volume of four liters for the solution. A third feedsolution (FS3) was prepared comprising 272.7 g 88% L-lactic acid, 2.4 g95% pyruvic acid, 20.7 g 10% w/v oxalic acid, and deionized water insufficient amount to attain a volume of four liters for the solution.

A 40 ml column was packed with Ionac A-365 weak base anion exchangeresin (Sybron Chemicals, Inc.), a basic immiscible extractant. IonacA-365 comprises porous polyacrylate gel bead structures, specificallycomprising acrylic divinylbenzene, with polyamine functional groups. Theresin packed in the column was washed with several bed volumes ofdeionized water. The packed column was capped off and placed on a ringstand in position for an upward flow of a feed stream.

An Eldex B-100-S-4 stainless steel reciprocating pump was used to adjustthe rate at which water, feed solution, and treatment fluids weredelivered to the column. The feed solutions, water and treatment fluidswere run through the column at temperatures of between about 19° C. and21° C. Four liters each of the lactic acid feeds FS 1 and FS2 and twoand a half liters of FS3 were pumped in sequence through the column overa period of more than 52 hours. The solutions were fed, one after theother, to the bottom of the column and eluent was collected from the topof the column. Eluent fractions from the feed solutions were collectedin the following order 6×30 ml, 2×120 ml, 29×240 ml, 1×40 ml, 8×240 ml,and 9×30 ml. The column was subsequently treated with an acidic solutionand a basic solution, and thus 4×40 ml acidic eluent fractions, 2×40 mldeionized water wash fractions, 4×40 ml caustic (e.g. basic) eluentfractions, and 2×40 ml deionized water wash fractions were collected, aswell.

Select eluent fractions were subsequently analyzed, undiluted, by HPLC.100 μl of an eluent fraction was injected for each HPLC run at a flowrate of 1.4 ml per minute. All analyses were conducted at roomtemperature between 19° C. and 21° C. The mobile phase comprised 10%acetonitrile and 0.085% H₃PO₄ in deionized water. The HPLC column was aJordi organic acid column 300 mm long and 7.8 mm in diameter. A UVdetection device (210 nm) was used to detect compounds as they elutedfrom the HPLC column. The pH values of certain eluent fractions weredetermined.

Following collection of the fractions described above, two 30 ml sampleswere collected at a flow rate approximately half of the rate used forthe previous fractions in order to determine if there were any kineticlimitations. Based on the data below, it appeared there were no kineticeffects.

Following the run of the feed stream, the column was regenerated. Thecolumn was washed with two bed volumes of deionized water, four bedvolumes of 1 N HCl, followed by another wash comprising two bed volumesof deionized water. The column was next washed using four bed volumes of1 N NaOH, followed by a wash of two bed volumes of deionized water. Feedsolution runs and regeneration of the column can be described using thefollowing chemical equations.

Feed Treatment:R₃N→R₃N:Hlactic→R₃N:HPyruvic-R₃N:HOxalic

Regeneration(1):R₃N:HPyruvic and R₃N:HOxalic+HCl→R₃N:HCl+pyruvic acidand oxalic acid

Regeneration(2):R₃N:HCl+NaOH→R₃N+H₂O+NaCl

A number of new peaks appeared in the acid and base elution fractionsand the water washes that followed them. It is possible that some ofthese peaks could be explained by reactions between pyruvic acid andoxalic acid in the resin phase.

Initially the resin was in the form of R₃N. As the feed solution was fedonto the column, the immiscible basic extractant complexed with all ofthe acids present in the feed stream. As more of the feed solution wasfed onto the column, the lactic acid that was complexed with theimmiscible basic extractant was displaced by pyruvic acid. As more feedsolution was fed onto the column, the pyruvic acid complexed with theimmiscible basic extractant was displaced by oxalic acid. Analysis ofHPLC traces of the eluent samples that were collected at the end of theflow of the feed solution over the column, reveals that oxalic acid wasstill being absorbed almost completely by the immiscible basicextractant at the end of the run. Breakthrough of pyruvic acid (pyruvicacid being displaced from the immiscible basic extractant) wasconsidered to have occurred when pyruvic acid in an eluent sample hadreached five percent of the gram/liter concentration in the feed. Fromthe HPLC data, this happened at 192 bed volumes.

Other data collected from the run are summarized in the tables below.

TABLE 2 Sample peak areas of HPLC traces for the feed solutions FS1 FS2FS3 Oxalic acid 1614 1576 1614 Pyruvic acid 1462 1251 1292 Lactic acid9503 7921 8285 Lactic dimer 3240 3544 3693 Lactic trimer 685 911 866

TABLE 3 Relevant pK_(a) values for the acids involved ACID pK_(a) HCL −2OXALIC ACID 1.27 PYRUVIC ACID 2.48 FORMIC ACID 3.75 LACTIC ACID 3.86ACETIC ACID 4.76

TABLE 4 A comparison of an eluent sample and the feed it came fromEluent fraction FS1 (FS1 after run on column) Oxalic acid 512 0 Pyruvicacid 639 2 Lactic acid 44199 47288 Lactic dimer 15070 14179 Lactictrimer 3188 2676

The pyruvic acid area corresponds to a level of about 2 ppm or 0.002gram/liter of pyruvic acid in the effluent.

Several impurities of unknown chemical composition that appeared in theHPLC traces of the feed solutions and eluent fractions are probably dueto resin impurities such as residual monomer from the resinmanufacturing and impurities present in the chemicals used to preparethe feed solution.

TABLE 5 The bed loading at breakthrough based on the theoretical bedcapacity of 3.5 meq per ml Millimoles per Species Method milliliter ofresin Oxalic acid from analytical data 1.09 Pyruvic acid from analyticaldata 1.27 Lactic acid, Lactic from theoretical 1.14 dimer and Lactic bedcapacity trimer

EXAMPLE 3 Batchwise Treatment of Overhead Distillate From Lactic AcidAzeotropic Distillation

A fermentation broth containing lactic acid had previously been purifiedby azeotropic distillation and other steps. This product contained 19ppm pyruvic acid and 260.9 g/L lactic acid. 4.5 ml of this material wastreated with about 1.2 ml of a weak base anion exchange resin, SybronIonac A-365, in the hydroxy form in a simple batch equilibriumadsorption. Approximately 38% of the lactic was removed. Remarkably, 90%of the pyruvic acid was removed leaving a product with less than 2 ppmpyruvic acid. A selectivity for pyruvic acid relative to lactic acid of14.73 was estimated.

Additionally, an unknown acid “strong impurity” with an elution time of7.27 minutes in this particular HPLC chromatographic analysis protocolwas observed to be 99% removed by this treatment, with a selectivity ofabout 54.

In the table below, a chromatographic response factor for the unknownpeak at 210 nanometer wavelength has been estimated based on refractiveindex data (not shown) suggesting that the species has a response factor4 times greater than that of pyruvic acid.

TABLE 6 Concentration Equilibrium % mole/liter Feed Solution LiquorResin K_(d) Selectivity Removal Peak 7.27 12.57 × 10⁻⁴ 0.13 × 10⁻⁴ 52.47 × 10⁻⁴ 401.21 176.51 99% Pyruvic Acid  0.22 × 10⁻⁴ 0.02 × 10⁻⁴  0.73 × 10⁻⁴ 33.49 14.73 90% Lactic Acid 2899.92 × 10⁻⁴  1805.11 ×10⁻⁴   4102.98 × 10⁻⁴ 2.27 38% Lactic Dimer 91.26 × 10⁻⁴ 0.55 × 10⁻⁴ 178.90 × 10⁻⁴ 4.11 1.81 52%

EXAMPLE 4 Continuous Treatment of Overhead Distillate from Lactic AcidAzeotropic Distillation

A fermentation broth containing lactic acid had previously been purifiedby azeotropic distillation and other steps.

This product did not contain detectable levels of pyruvic acid, 743 g/Llactic acid, and an estimated 0.22 gram/liter of the unknown peak at7.27 minutes.

A sample of 10.0 ml of this material was treated with about 2.4 ml of aweak base anion exchange resin, Sybron Ionac A-365, in the hydroxy formin a continuous column contacting mode.

Approximately 15% of the lactic was removed. Remarkably, 98% of theunknown strong impurity was removed, giving a lactic acid product ofless than 4 ppm of this strong impurity.

This example teaches the effectiveness of the current invention forremoving strong impurities whose exact identity is unknown.

EXAMPLE 5 Continuous Treatment of Low pH Fermentation Broths

Two low pH lactic acid fermentation broths were prepared (each was about5.5 liters in volume) and combined. The low pH broth was treated withSAC (strongly acid cation exchange) resin (>0.1 moles of resin/molelactic acid) and WBA (weakly basic anion exchange) resin (˜0.03 molesresin/mole lactic acid). The SAC resin was packed in two 37 mm×450 mmcolumns, each 480 ml, operated in tandem and the WBA resin was in a two15 mm×300 mm columns, of 53 ml each, in tandem.

After treatments, the product was found to contain very low levels ofpyruvic acid, less than found in samples of lactic acid supplied by ADMand Pfanstiehl.

TABLE 7 All concentrations in ppm except for lactic acid, in gram/literImpurity Effluent Effluent Effluent Effluent Acid Species Type Feed 1 23 4 Lactic acid Product 60.76 40.28 62.64 63.16 62.92 gram/L gram/Lgram/L gram/L gram/L Pyruvic acid Strong 270 <20 <20 <20 <20 HCL Strong117 <20 <20 <20 <20 H₂SO₄ Strong 125 <20 <20 <20 <20 H₃PO₄ Strong 690<20 <20 <20 <20 Malic acid Weak 32 19 39 26 23 Acetic acid Weak 15 <1012 14 15 Succinic acid Weak 48 <20 41 33 30

We see from the tabulated data that both inorganic and organic acids canbe strong impurities can be removed with good effectiveness despite thehigh level of lactic acid. A flowrate of 1.4 BV/hour for the SAC and 6BV/hour for the WBA was used.

EXAMPLE 6 Continuous Treatment of 11 Liters of Fermentation Broth

The resins used in example 5 were regenerated and the treatment wasrepeated with a fresh batch of fermentation material. A typical effluentfraction collected during the run, was 62.8 gram/liter lactic acid and12.4 ppm pyruvic acid.

EXAMPLE 7 Continuous Treatment of 11 Liters of Fermentation Broth

The resins used in example 6 were regenerated and the treatment wasrepeated with a fresh batch of fermentation material. Two fractions werecollected. After ion exchange treatment, each fraction was concentratedby evaporation to nearly 28% w/w lactic acid. Prior to evaporation, theconcentrations are as shown in Table 8.

TABLE 8 Gram/liter concentrations Feed Fraction Pyruvic Acid 0.145 0.005Lactic Acid 62.000 62.000 Malic Acid, Peak 9.00 0.130 0.030

It can be seen that the pyruvic acid level is lowered from 145 mg/literto 0.5 mg/liter. This remarkable reduction in pyruvic acid level ofnearly 300-fold was achieved with only minor loss of lactic acid.

The anion resin was regenerated and it was found that 0.64% of the totalfeed mass of lactic acid had been adsorbed on the ion exchange resinwith the pyruvic acid. In this case, the strong acids H₂SO₄ and H₃PO₄are present in the feed and also act as displacing acids

Additional displacing acids can be used to regenerate the resin andselectively displace the additional lactic acid in preference to thepyruvic acid.

The anion ion exchange resin was regenerated and found that the ratio ofM lactic acid to M pyruvic acid on the resin was 19.34. This is asexpected due to the selectivity limit of the resin. This load cannot beexceeded.

EXAMPLE 8 Comparison of Liquid Immiscible Amine and Solid Amine IonExchanger for Sequence of Strong Impurities

A acid mixture solution was prepared with 52.78 g/L lactic acid 0.2 to0.3 g/L each the acid impurities listed in the table.

An 8 ml aliquot of the acid mixture was equilibrated with 1.0 ml ofSybron Ionac A-365 weak base anion resin and the selectivities measuredrelative to lactic acid.

An separate aliquot of 5 ml of the acid mixture was separatelyequilibrated with 5 ml of extractant Y, consisting of 1.0 molartrilaurylamine and 1.0 molar dodecanol, with dodecane as the diluent.

TABLE 9 Selectivity of Ionac Selectivity of Extractant ‘Y’ A-365 forimpurity for impurity relative to lactic Species relative to lactic acidacid Pyruvic Acid 20.99 L-Malic Acid 198.8 3.32 Formic Acid 4.01 3.06Acetic Acid 0.169 0.42 Propionic Acid 0.101 1.42 n-butyric acid 0.1005.74 Iso-butyric acid 0.072 6.53

EXAMPLE 9 U38 Strong Base Anion Resin to Selectively Remove StrongImpurities Followed by Displacement with Displacing Acid

Amberlite IRA-93 strong base anion resin was prepared in a 62 ml columnand regenerated to give the hydroxide form. The resin was used to treatan excess of concentrated lactic acid fermentation broth that hadpreviously already been treated with a cation and weak base anion resin.

The resin was displaced using 1 N H₂SO₄. Five fractions were collected.The first two fractions contained significant weak impurities, aceticacid and formic acid, as well as lactic acid. The latter fractionscontained significant pyruvic acid as well as some lactic acid.

EXAMPLE 10 Calculations of the Effect of Varying Pyruvic Acid Levels inthe Feed on Removal and Displacement Effectiveness

The feed solution had a ratio of lactic acid moles to pyruvic acid molesof 117:1. After treatment with an amine immiscible basic extractant, theproduct had a comparable ratio of 486:1. Thus, the lactic acid had beenpurified by selective removal of the pyruvic acid. The extract contained5.6% mole of the lactic acid that was present in the feed, and thisdiminished the yield.

In general, more stages or a longer extraction sequences could notimprove this situation very much. This was because the feed already hadquite low levels of pyruvic acid impurity, yet the amine only had alimited selectivity for pyruvic acid over lactic acid of 18-fold.

The values used for the distribution coefficient for each of the speciesinto the amine phase were assumed to be constant. This was reasonable asconcentrations do not change significantly over the course of theextraction. The distribution coefficients for each species were assumedto be independent, which is also reasonable as the amine was onlylightly loaded in this case. At low loadings, the amine extract complexwas probably dominated by the formation of 1:1 complexes of pyruvic acidwith the amine. The energy of these complexes and the distribution wasroughly correlated with the pK_(a) of the acid. The amine used here wasprobably not highly enhanced. For this example, it was assumed that theK_(d) for pyruvic acid into the amine phase, was the ratio of pyruvicacid mole/liter of acid free amine phase to pyruvic acid mole/liter ofacid free water phase. The distribution coefficient for pyruvic acid wasassumed to be 5.0 and that for lactic acid to be 0.278, giving aselectivity of 18.0.

TABLE 10 Effect of feed concentration of pyruvic acid when the samelactic acid concentration (0.66 M) is used. Pyruvic acid 1000 500 300100 ppm concentration initially in feed Volume of immiscible 10.66 10.5710.53 10.49 liter basic extractant required Bed volume 93.84 94.64 94.9695.29 liter treated Lactic acid losses to 3.9% 4.7% 5.1% 5.4% extractY_(pyruvic), ratio M 30.5% 15.4% 9.2% 3.1% pyruvic acid to M lactic acidon the ion exchange bed

Effective pyruvic acid level in the effluent is very low after a secondbed volume, due to the large number of stages in ion exchange. A levelof 2 ppm pyruvic acid was observed in practice. Assuming that the bedhad a selectivity of 18:1 for pyruvic over lactic acid, then duringregeneration it was possible to separate the lactic acid from thepyruvic acid nearly quantitatively in all cases where the ion loadfraction on the bed, y_(pyruvic), was greater than 100/18=5.56% w/w. Ifthe loading was less than this, then a subsequent additional ionexchange treatment would be required for that effluent. The exampleabove is for the case of full desorption of all the species on the ionexchanger without fractionation of the eluents, which is desirable.

EXAMPLE 11 The Calculations of Example 10 are Repeated, But the LacticAcid Concentration Is Varied

TABLE 11 Effect of varying concentration of lactic acid concentration infeeds having the same pyruvic acid concentration. Pyruvic acid 500 500500 ppm concentration initially in feed Volume of immiscible 10.57 12.7915.96 liter basic extractant required Bed volume treated 94.64 78.1962.64 liter Lactic acid losses to 4.74% 4.88% 5.02% extract Y_(pyruvic)15.4% 12.7% 10.2% Lactic acid 0.66 0.80 1.00 mol/L concentrationinitially in feed

For a given level of pyruvic acid, as the lactic acid concentrationincreases, the fractional loading of lactic acid on the bed decreased.This makes obtaining separate fractions of lactic acid and pyruvic acidmore difficult. The preceding description of specific embodiments of thepresent invention is not intended to be a complete list of everypossible embodiment of the invention. Persons skilled in this field willrecognize that modifications can be made to the specific embodimentsdescribed here that would be within the scope of the present invention.

What is claimed is:
 1. A process for purifying an organic acid,comprising: providing an aqueous feed stream comprising a productorganic acid and at least one strong contaminant, wherein the molarconcentration of the product organic acid is at least 20 times greaterthan the molar concentration of the strong contaminant; contacting theaqueous feed stream with a first immiscible basic extractant that has aselectivity for the strong contaminant relative to the product organicacid that is greater than about 3, whereby the majority of the strongcontaminant and less than about 33 wt % of the product organic acidbecome complexed with the first immiscible basic extractant; separatingthe complexed first immiscible basic extractant from the aqueous stream,thereby producing a first effluent stream that comprises product organicacid and that has a greater ratio of product organic acid to strongcontaminant than the aqueous feed stream did; and contacting thecomplexed first immiscible basic extractant with a displacing acid,wherein the first immiscible basic extractant has a greater affinity forthe displacing acid than it does for the strong contaminant or theproduct organic acid, and wherein the displacing acid is present insufficient amount to cause product organic acid to be selectivelydisplaced from the complexed first immiscible basic extractant, thereby,over time, producing a second effluent stream that comprises a majoramount of product organic acid and displacing strong contaminant fromthe complexed first immiscible basic extractant, thereby followingproduction of the second effluent stream with the production of a thirdeffluent stream that comprises a major amount of strong contaminant andthat is separated from the second effluent stream; wherein the totalamount of product organic acid present in the first effluent stream andin the second effluent stream is at least about 900/0 by weight of theproduct organic acid that was present in the feed stream.
 2. The processof claim 1, wherein the total amount of product organic acid present inthe first effluent stream and in the second effluent stream is at leastabout 98% by weight of the product organic acid that was present in thefeed stream.
 3. The process of claim 1, wherein the molar concentrationof the product organic acid is at least 90 times greater than the molarconcentration of the strong contaminant.
 4. The process of claim 1,wherein the molar concentration of the product organic acid is at least500 times greater than the molar concentration of the strongcontaminant.
 5. The process of claim 1, wherein the molar concentrationof the product organic acid is at least 1000 times greater than themolar concentration of the strong contaminant.
 6. The process of claim1, wherein the selectivity is at least about
 15. 7. The process of claim1, wherein the selectivity is at least about
 25. 8. The process of claim1, wherein the selectivity is at least about
 100. 9. The process ofclaim 1, wherein the strong contaminant is an organic acid that has apK_(a) that is lower than that of the product organic acid.
 10. Theprocess of claim 1, wherein the product organic acid is lactic acid. 11.The process of claim 10, wherein the strong contaminant has a pK_(a)less than about 3.46.
 12. The process of claim 10, wherein the strongcontaminant is selected from the group consisting of pyruvic acid,oxalic acid, citraconic acid, citric acid, and mixtures thereof.
 13. Theprocess of claim 10, wherein the selectivity is greater than about 25.14. The process of claim 10, wherein the molar concentration of thelactic acid in the feed stream is at least 300 times greater than themolar concentration of the strong contaminant in the feed stream, andthe selectivity is greater than about
 500. 15. The process of claim 1,wherein the first immiscible basic extractant comprises a weak base ionexchange resin.
 16. The process of claim 1, wherein the displacing acidis selected from the group consisting of HCl, H₂SO₄, H₃PO₄, oxalic acid,and trifluoroacetic acid.
 17. The process of claim 1, wherein the feedstream, the first effluent stream, and the second effluent streamfurther comprise a weak contaminant, the process further comprising:combining the first effluent stream and the second effluent stream toform a combined product organic acid stream; contacting the combinedproduct organic acid stream with a second immiscible basic extractant,whereby the majority of the product organic acid becomes complexed withthe second immiscible basic extractant; and separating the complexedsecond immiscible basic extractant from the stream, thereby producing afourth effluent stream that comprises the majority of the weakcontaminant that was present in the combined product organic acidstream.
 18. The process of claim 17, further comprising: contacting thefourth effluent stream with a third immiscible basic extractant that hasa greater affinity for the product organic acid than for the weakcontaminant, whereby the majority of the product organic acid that ispresent in the fourth effluent stream becomes complexed with the thirdimmiscible basic extractant; separating the complexed third immisciblebasic extractant from the stream, thereby producing a fifth effluentthat comprises the majority of the weak contaminant that was present inthe combined product organic acid stream; contacting the complexedsecond immiscible basic extractant and the complexed third immisciblebasic extractant with one or more displacing acids, thereby displacingproduct organic acid therefrom in one or more additional effluentstreams.
 19. The process of claim 17, wherein the second immisciblebasic extractant comprises a weak base ion exchange resin comprising atertiary amine moiety.
 20. The process of claim 17, wherein the productorganic acid is lactic acid and the weak contaminant is an organic acidhaving a pK_(a) greater than about 4.26.
 21. The process of claim 20,wherein the weak contaminant is an organic acid selected from the groupconsisting of propionic acid, butyric acid, malonic acid, succinic acid,and mixtures thereof.
 22. The process of claim 1, further comprising:contacting the third effluent stream with an additional immiscible basicextractant that has a greater affinity for the strong contaminant thanfor the product organic acid, whereby the majority of the strongcontaminant present in the third effluent stream becomes complexed withthe additional immiscible basic extractant; separating the complexedadditional immiscible basic extractant from the remaining stream,thereby producing an additional effluent that comprises the majority ofthe product organic acid that was present in the third effluent.
 23. Theprocess of claim 1, wherein the feed stream comprises a fermentationbroth.
 24. The process of claim 1, wherein the first immiscible basicextractant comprises a solid basic extractant, contacting the aqueousfeed stream with the first immiscible basic extractant is performed in apacked bed, and the ratio of molar concentration of product organic acidto molar concentration of strong contaminant in the second effluentstream is within about 10% of the selectivity.
 25. The process of claim1, wherein the first immiscible basic extractant comprises a liquidbasic extractant, contacting the aqueous feed stream with the firstimmiscible basic extractant is performed as a multistage process, andthe ratio of product organic acid to strong contaminant is within about10% of the selectivity.
 26. The process of claim 1, wherein the aqueousfeed stream comprises more than one strong contaminant, and wherein atleast one of the strong contaminants is a displacing acid.
 27. Theprocess of claim 1, wherein the feed stream and the first effluentstream further comprise a weak contaminant, the process furthercomprising: processing the first effluent stream by at least one ofextraction or distillation, whereby at least two fractions are produced,a purified product organic acid fraction comprising between about 90%and 99.5% by weight of the product organic acid that was present in thefeed stream, and a weak contaminant fraction comprising product organicacid and weak contaminant, wherein the ratio of molar concentration ofproduct organic acid to molar concentration of weak contaminant in theweak contaminant fraction is less than the ratio of molar concentrationof product organic acid to molar concentration of weak contaminant inthe feed stream; contacting the weak contaminant fraction with a thirdimmiscible basic extractant that has a selectivity for the productorganic acid relative to the weak contaminant that is greater than about3, whereby the majority of the product organic acid and less than about33 wt % of the weak contaminant become complexed with the thirdimmiscible basic extractant; separating the complexed third immisciblebasic extractant from the aqueous stream, thereby producing an effluentstream that comprises weak contaminant and that has a greater ratio ofweak contaminant to product organic acid than the aqueous feed streamdid; and contacting the complexed third immiscible basic extractant witha displacing acid, wherein the third immiscible basic extractant has agreater affinity for the displacing acid than it does for the productorganic acid or the weak contaminant, and wherein the displacing acid ispresent in sufficient amount to cause product organic acid and weakcontaminant to be displaced over a period of time from the complexedthird immiscible basic extractant, thereby producing a weak contaminanteffluent stream that comprises a major amount of weak contaminant and aproduct organic acid stream that comprises a major amount of the productorganic acid.
 28. The process of claim 27, wherein the purified productorganic acid fraction comprises between about 95% by weight and 99.5% byweight of the product organic acid that was present in the feed stream.29. The process of claim 1, wherein the first immiscible basicextractant has previously been used to treat a solution comprising theproduct organic acid and at least one weak contaminant.
 30. A processfor purifying lactic acid, comprising: providing an aqueous feed streamcomprising lactic acid and at least one strong contaminant acid having apK_(a) less than about 3.46, wherein the molar concentration of lacticacid is at least 20 times greater than the molar concentration of thestrong contaminant acid; contacting the aqueous feed stream with a firstbasic ion exchanger that has a greater affinity for the strongcontaminant acid than for lactic acid, whereby the majority of thestrong contaminant acid and some lactic acid become complexed with thefirst basic ion exchanger; separating the complexed first basic ionexchanger from the aqueous stream, thereby producing a first effluentstrewn that comprises lactic acid and that has a greater ratio of lacticacid to strong contaminant acid than the aqueous feed stream did; andcontacting the complexed first basic ion exchanger with a displacingacid, wherein the first basic ion exchanger has a greater affinity forthe displacing acid than it does for the strong contaminant acid orlactic acid, whereby lactic acid is selectively displaced from thecomplexed first basic ion exchanger, thereby, over time, producing asecond effluent stream that comprises a major amount of lactic acid anddisplacing strong contaminant from the complexed first basic ionexchanger, thereby following production of the second effluent streamwith the production of a third effluent stream that comprises a majoramount of strong contaminant acid and that is separated from the secondeffluent stream; wherein the total amount of lactic acid present in thefirst effluent stream and the second effluent stream is at least about90% by weight of the lactic acid that was present in the feed stream.31. The process of claim 30, wherein the molar concentration of lacticacid is at least 90 times greater than the molar concentration of thestrong contaminant acid in the aqueous feed stream.
 32. The process ofclaim 30, wherein the molar concentration of lactic acid is at least 500times greater than the molar concentration of the strong contaminantacid in the aqueous feed stream.
 33. The process of claim 30, whereinthe molar concentration of lactic acid is at least 1000 times greaterthan the molar concentration of the strong contaminant acid in theaqueous feed stream.
 34. The process of claim 30, wherein the strongcontaminant acid is selected from the group consisting of pyruvic acid,oxalic acid, citraconic acid, citric acid, and mixtures thereof.
 35. Theprocess of claim 30, wherein the molar concentration of lactic acid inthe feed stream is at least 100 times greater than the molarconcentration of the strong contaminant acid in the feed stream, and theselectivity is greater than about
 250. 36. The process of claim 35,wherein the molar concentration of the product organic acid in the feedstream is at least 300 times greater than the molar concentration of thestrong contaminant acid in the feed stream, and the selectivity isgreater than about
 500. 37. The process of claim 30, wherein the firstbasic ion exchanger comprises a weak base ion exchange resin.
 38. Theprocess of claim 30, wherein the first basic ion exchanger has anaffinity for the displacing acid that is at least 10 times greater thanits affinity for pyruvic acid.
 39. The process of claim 30, wherein thedisplacing acid is selected from the group consisting of HCl, H₂SO₄, andH₃PO₄.
 40. The process of claim 30, wherein the feed stream, the firsteffluent stream, and the second effluent stream further comprise a weakcontaminant organic acid having a pK_(a) greater than about 4.26, andthe process further comprises: combining the first effluent stream andthe second effluent stream to form a combined lactic acid stream;contacting the combined lactic acid stream with a second basic ionexchanger, whereby the majority of the lactic acid becomes complexedwith the second basic ion exchanger; and separating the complexed secondbasic ion exchanger from the stream, thereby producing a fourth effluentstream that comprises the majority of the weak contaminant that waspresent in the combined lactic acid stream.
 41. The process of claim 40,further comprising: contacting the fourth effluent stream with a thirdbasic ion exchanger that has a greater affinity for lactic acid than forthe weak contaminant acid, whereby the majority of the lactic acid thatis present in the fourth effluent stream becomes complexed with thethird basic ion exchanger; separating the complexed third basic ionexchanger from the stream, thereby producing a fifth effluent thatcomprises the majority of the weak contaminant acid that was present inthe combined lactic acid stream; contacting the complexed second basicion exchanger and the complexed third basic ion exchanger with one ormore displacing acids, thereby displacing lactic acid therefrom in oneor more additional effluent streams.
 42. The process of claim 40,wherein the second immiscible basic extractant comprises a weak base orstrong base ion exchange resin.
 43. The process of claim 40, wherein theweak contaminant is an organic acid selected from the group consistingof propionic acid, butyric acid, malonic acid, succinic acid, andmixtures thereof.
 44. The process of claim 30, further comprising:contacting the third effluent stream with an additional basic ionexchanger that has a greater affinity for the strong contaminant acidthan for lactic acid, whereby the majority of the strong contaminantacid present in the third effluent stream becomes complexed with theadditional basic ion exchanger; separating the complexed additionalbasic ion exchanger from the stream, thereby producing an additionaleffluent that comprises the majority of the lactic acid that was presentin the third effluent.
 45. The process of claim 30, wherein the firsteffluent stream and the second effluent stream collectively have a ratioof molar concentration of lactic acid to molar concentration of strongcontaminant that is greater than
 300. 46. The process of claim 45,wherein the first effluent stream and the second effluent streamcollectively have a ratio of molar concentration of lactic acid to molarconcentration of strong contaminant that is greater than about 1,000.47. The process of claim 30, wherein at least 98% by weight of thelactic acid present in the feed stream is recovered in the firsteffluent stream and the second effluent stream.
 48. The process of claim30, wherein the aqueous feed stream comprises or is obtained from afermentation broth.
 49. The process of claim 48, wherein the aqueousfeed stream comprises no more than about 0.15 moles of cations selectedfrom the group consisting of Ca, Mg, Na, Fe, Zn, Zr, and Li, per mole oflactic acid; no more than about 0.05 moles of anions selected from thegroup consisting of Cl, SO₄, PO₄, and NO₃, per mole of lactic acid; normore than about 0.03 mole of strong acid contaminants selected from thegroup consisting of pyruvic acid, oxalic acid, citraconic acid, andcitric acid, per mole of lactic acid; and no more than about 0.02 moleof weak acid contaminants selected from the group consisting ofpropionic acid, butyric acid, malonic acid, and succinic acid, per moleof lactic acid.
 50. A process for purifying lactic acid, comprising:providing an aqueous fermentation broth comprising lactic acid andpyruvic acid, wherein the molar concentration of the lactic acid is atleast 20 times greater than the molar concentration of pyruvic acid;removing cells from the broth to form an aqueous feed stream; contactingthe aqueous feed stream with means for complexing pyruvic acid, themeans having a greater affinity for pyruvic acid than for lactic acid,whereby the majority of the pyruvic acid and some lactic acid formcomplexes therewith; separating the complexes from the aqueous stream,thereby producing a first effluent stream that comprises lactic acid andthat has a greater ratio of lactic acid to pyruvic acid than the aqueousfeed steam did; and contacting the complexes with means for displacinglactic acid and pyruvic acid therefrom wherein the lactic acid isselectively displaced from the complexes thereby, over time, producing asecond effluent stream that comprises a major amount of lactic acid anddisplacing pyruvic acid from the complexes, thereby following productionof the second effluent stream with the production of a third effluentstream that comprises a major amount of pyruvic acid and that isseparated from the second effluent stream; wherein the total amount oflactic acid present in the first effluent stream and the second effluentstream is at least about 98% by weight of the lactic acid that waspresent in the feed stream.
 51. A process for purifying an organic acid,comprising: providing an aqueous feed stream comprising a productorganic acid and at least one strong contaminant, wherein the molarconcentration of the product organic acid is at least 20 times greaterthan the molar concentration of the strong contaminant; contacting theaqueous feed stream with a first immiscible basic extractant that has aselectivity for the strong contaminant relative to the product organicacid that is greater than about 3, whereby the majority of the strongcontaminant and less than about 33 wt % of the product organic acidbecome complexed with the first immiscible basic extractant; separatingthe complexed first immiscible basic extractant from the aqueous stream,thereby producing a first effluent stream that comprises product organicacid and that has a greater ratio of product organic acid to strongcontaminant than the aqueous feed stream did; and exposing the complexedfirst immiscible basic extractant to at least one of (1) a change intemperature, (2) a change in solvent concentration, or (3) a change indisplacing agent concentration, wherein exposure causes product organicacid to be selectively displaced from the complexed first immisciblebasic extractant, thereby over a period of time producing a secondeffluent stream that comprises a major amount of product organic acidand displacing strong contaminant from the complexed first immisciblebasic extractant, thereby following production of the second effluentstream with the production of a third effluent stream that comprises amajor amount of strong contaminant and that is separated from the secondeffluent stream; wherein the total amount of product organic acidpresent in the first effluent stream and in the second effluent streamis at least about 90% by weight of the product organic acid that waspresent in the feed stream.